Pregnancy is a specific event that often results in decreased physical activity for women; thereby placing this special population at increased risk for several chronic diseases and premature mortality. The existing research examining exercise during pregnancy is limited, however, with respect to the following 3 important factors: (a) cross-sectional assessments that do not capture the specific physical and psychological demands of each trimester; (b) nonstandardized and unvalidated measures of exercise with pregnant women; and (c) nontheoretical studies of exercise determinants. Before effective physical activity interventions are implemented with pregnant women, it is necessary to prospectively examine, during each trimester, women's exercise beliefs and behaviors using standardized measures of exercise behavior within a theoretical framework. This approach will provide the theoretical basis to implement exercise interventions specific to the physical and psychological demands of each trimester. The theory of planned behavior is one of the most influential and successful theories for understanding and predicting exercise intention and behavior, and will thus serve as the theoretical framework to prospectively determine the exercise beliefs and behaviors for this special population. The specific aims of this application are: (1) Determine the normative, control, and behavioral beliefs of exercise during each trimester. (2) Determine whether: (a) the beliefs predict attitude, perceived behavioral control, and subjective norm during each trimester; (b) intention and perceived behavioral control predict exercise behavior during each trimester; and (c) attitude, perceived behavioral control, and subjective norm predict exercise intention during each trimester. (3) Examine physical activity levels during each trimester. And (4) explore potential mediators and moderators of the theory of behavior. [unreadable] [unreadable] [unreadable]